1. Field of the Invention
The present invention relates to a battery pack.
2. Description of the Related Art
Recently, compact and lightweight portable electrical/electronic devices such as cellular phones, notebook computers, camcorders and the others have been widely developed and manufactured. These portable electrical/electronic devices are equipped with a battery pack so as to be operated without an additional power source. A battery pack typically uses a rechargeable secondary battery so as to be more economical. The secondary battery may be, for example, a nickel-cadmium (Ni—Cd) battery, a nickel-metal hybrid (Ni-MH) battery, a lithium (Li) battery and a lithium-ion (Li-ion) secondary battery. Particularly, Li-ion secondary batteries have an operation voltage about three times higher than Ni—Cd batteries or Ni-MH batteries typically used to power portable electronic equipment. In addition, Li-ion secondary batteries are widely used because they have a high energy density per unit weight.
The secondary battery primarily uses a lithium-based oxide as a cathode active material and carbon materials as an anode active material. Generally, secondary batteries are classified into a liquid electrolyte battery and a polymer electrolyte battery according to a kind of electrolyte used therein. A battery using a liquid electrolyte is referred to as a lithium-ion battery and a battery using a polymer electrolyte is referred to as a lithium-polymer battery.
The secondary battery may be formed by electrically coupling a bare cell to a protection circuit. The bare cell is formed by sealing a can containing an electrode assembly and electrolyte. The bare cell is charged and discharged through a chemical reaction and the protection circuit controls charge/discharge of the bare cell. Accordingly, the bare cell is protected from over-charge and over-discharge by the protection circuit.
Typically in a secondary battery, the bare cell is electrically coupled to the protection circuit to reduce electrical resistance between the bare cell and the protection circuit, and thus charge/discharge efficiency of the bare cell is increased. In other words, when the electrical resistance between the bare cell and the protection circuit increases, the charge/discharge efficiency of the bare cell is reduced.
The secondary battery is assembled into a battery pack by integrating the bare cell with the protection circuit so as to be configured to be installed on an electronic device, and then a reliability test is performed to estimate stability due to impact. It has been found that the electrical resistance between the bare cell and the protection circuit in the battery pack increases upon external impact. The increase in electrical resistance may be due to an increase in contact resistance between the bare cell and the protection circuit caused by separation of the bare cell from the protection circuit due to the external impact.